Under the Blue Moon Tonight

I always get a little buzz when I discover the links between origins of words and expressions. One such gem occurred this week, when I heard about the imminent blue moon, which will occur very soon on the 31st July.

A blue moon is the second full moon in a calendar month, an event that occurs once every 2.7 years – not very often. Hence the expression ‘once in a blue moon’, right? Not so fast. A little research reveals history is not so linear.

Blue Moon: the lunar event

This definition of the lunar event itself is actually quite modern and can be traced back to 1950 when amateur astrologer James Hugh Pruett mistakenly coined the term in Sky and Telescope. He had lifted the term from a 1937 issue of the North American publication Maine Farmers’ Almanac. Closer examination of the almanac reveals the term was actually used to describe the third out of four full moons in a season (usually there are only 3). This definition was based on the seasons of a ‘tropical year’ which is dependent on various ecclesiastical dates calculated according to the Gregorian calendar. So complicated was this definition that Pruett misinterpreted it as the 2nd full moon in a calendar month. It seems to have stuck!

Once in a Blue Moon: the expression

The expression meaning that something happens rarely, follows another story altogether. The earliest found reference to the colour of the moon was in an anti-clerical pamphlet published in 1528 by William Roy and Jeremy Barlowe [good name!]: “Yf they say the mone is blewe/We must believe that it is true.” This meaning, that lay people were expected to believe whatever the Church said, derives from the idea that the moon appearing to be the colour blue is something that would never happen; something of an absurdity. However blue moons do happen! For two years after the eruption of Krakatoa in Indonesia in 1883, sunsets turned green and the moon appeared blue all over the world.

The Missing Link

Language adapts, and we know that by 1821 the usage of the expression in the media in London had changed from something that never happens, to something that happens very rarely. This happened to be around the same time that the Maine Farmers’ Almanac first used it to describe the extra full moon of a season (1921). So although they are separate, it is still technically possible that the older definition of a blue moon event used by the almanac, did originally come from the expression ‘once in a blue moon’. We may never know for sure.

La Lune Bleue: the French version of events

So what do the French have to say about all this? Not surprisingly, the term ‘la lune bleue’ has been adopted from English to describe the lunar event as the 2nd full moon of a calendar month. However, the expression meaning something that happens very rarely, has not travelled across the channel, nor the Atlantic Ocean,= for that matter. They have their own expression for this: ‘tous les trente-six du mois’ (every 36th day of the month). This illuminates even further that the expression and the modern day term describing the lunar event evolved separately.

Curiously the French Wikipedia page on the subject also describes a blue moon as the 13th full month in a year. Really, I’m beginning to get the impression this is all just semantics!

For the keen astrologer the blue moon about to occur this Friday 31st July takes on another significance altogether.